Thin films are widely used in the electronic device industry. As the trend for miniaturization of electronic devices moves into the nanoscale domain, the reliability of thin films becomes an increasing concern. Building on the author¡¯s previous book, ¡°Electronic Thin Film Science¡± by Tu, Mayer, and Feldman, and based on a graduate course at UCLA given by the author, this new book focuses on reliability science and the processing of thin films. Early chapters address fundamental topics in thin-film processes and reliability, including deposition, surface energy, and atomic diffusion, before moving on to explain irreversible processes in interconnect and packaging technologies systematically (Chapter 10). Describing electromigration, thermomigration, and stress-migration, with a closing chapter dedicated to failure analysis, linking statistical and physical examinations (Chapter 15), the reader will come away with a complete theoretical and practical understanding of electronic thin-film reliability. Kept mathematically simple, with real-world examples, this book is ideal for graduate students, researchers, and practitioners.

The trend in consumer electronic products will be more and more wireless, portable, and handheld. To manufacture these multi-functional products, high-density circuit interconnections between a Si chip and its substrate are needed. The demand of flip chip solder joint technology is growing rapidly, in which an area array of solder bumps is used to join a chip to its substrate. Flip chip technology is the only technology that can provide a large number of such interconnections with reliability. Solder joints are ubiquitous in electronic products.

Due to environmental concern of toxicity of Pb-based solders, European Union Parliament has a directive to ban the use of Pb-based solders in consumer products on July 1st, 2006. The application of Pb-free solder joints to a wide range of devices is urgent, so the R&D of Pb-free solders for electronic manufacturing is very active at the moment. While solder joint technology is mature, Pb-free solder technology is not, hence their reliability has to be assured. For example, electrical shorting due to Sn whiskers, electrical opening due to electromigration, and joint fracture due to dropping of handheld devices to the ground are challenging reliability problems in the application of Pb-free solders. To solve these problems in a largely technology based manufacturing industry, scientific understanding and solutions are required. The copper-tin reaction is essential in the formation of a solder joint and the failure of a joint is due to externally applied forces as in electromigration. A fundamental understanding of the copper-tin reaction and the effect of external forces on solder joint reliability is critical and is emphasized in this book.

There are two themes in this book. The first is the copper-tin reaction as a function of time and temperature, and the second is the effect of external forces on the reaction. Actually the second theme also emphasizes phase transformations under an inhomogeneous boundary condition. Typically, metallurgical phase transformations occur under constant temperature and constant pressure so that Gibbs free energy is minimized. However, in thermomigration or stress-migration (creep) of a solder joint, the temperature or the pressure is not constant because there exists a temperature gradient or a stress gradient to drive the phase change, so an equilibrium state with a minimum free energy will not be reached. In electromigration, a potential gradient exists across the sample too.

The contents of the book are divided into two parts. Part I, from Chapter 2 to 7, covers copper-tin reactions, and Part II, from Chapter 8 to 12, covers electromigration and thermomigration of solder joints.

Chapter 1 is an overview of flip chip technology. Why it is important and what are the known reliability problems are explained. The future trend in electronic packaging technology and its effect on solder joint technology is given. Chapter 2 is about wetting reactions between molten eutectic solder and bulk Cu foils. The unique morphology of scallop-type Cu-Sn intermetallic compound formation is emphasized and analyzed. Chapter 3 is about Cu-Sn reactions in thin films. Thin film reactions are important since most metallization on Si devices to be joined by solder is in thin film form. Spalling of thin film intermetallic compounds is a unique reliability phenomenon. Chapter 4 is on solder reaction in a flip chip configuration in which the reaction occurs on two interfaces. The two interfacial reactions interact with each other and the interaction is a reliability issue. Chapter 5 presents a theoretical analysis of flux-driven ripening of scallop-type growth of Cu-Sn intermetallic compounds under the constraint of a constant surface area. Theoretically derived and experimentally measured distribution functions of scallops are compared. Chapter 6 is about spontaneous Sn whisker growth which is a creep phenomenon. The necessary and sufficient conditions of whisker growth are discussed, and how to conduct an accelerated test of Sn whisker growth and how to prevent its growth are presented. Chapter 7 discusses briefly solder reactions on nickel, palladium, and gold surfaces. In addition to copper, these metals are used as under-bump metallization in devices. Chapter 8 covers the fundamentals of electromigration and what is different between electromigration in solder alloys and in Al or Cu interconnects. Why electromigration in solder joints has only recently become a reliability problem is explained. Chapter 9 is about the unique behavior of electromigration in flip chip solder joints, especially the effect of current crowding. It is a key chapter of the book. The unique failure model due to pancake-type void formation at the cathode contact interface is given. Chapter 10 is about the interaction between electrical and chemical forces in solder joints. The polarity effect of electromigration on intermetallic compound formation at the cathode and the anode interfaces of a solder joint is presented. Chapter 11 describes the interaction between electrical and mechanical forces. An accidental drop to the ground is the most frequent cause of failure of portable devices. Impact test and drop test of solder joints are analyzed, and the effect of electromigration on these tests is discussed. Chapter 12 is about thermomigration in solder joints, and the interaction between electrical and thermal forces is analyzed. Microstructure instability in a eutectic two-phase structure driven by a temperature gradient is addressed

I started solder research in 1965, when I began my Ph.D. dissertation on cellular precipitation of Sn lamellae in SnPb alloys. However, the contents of this book are based on thirteen Ph. D. dissertations finished in the University of California at Los Angeles since 1996, supported by National Science Foundation (Dr. Bruce MacDonald), Semiconductor Research Corporation (Dr. Harold Hosack), and several microelectronic companies (especially Dr. Paul A. Totta of IBM East Fishkill, NY, Dr. Fay Hua of Intel, Santa Clara, CA, Dr. Luu Nguyen at NSC, Santa Clara, CA, Dr. Darrel Frear at Freescale, Phoenix, AZ, and Dr. Yi-Shao Lai at ASE, Taiwan). The dissertations of H. K. Kim, Patrick Kim, Chih Chen, Cheng-Yi Liu, Woo-Jin Choi, Hua Gan, Albert T. Wu, Emily Shengquan Ou, Minyu Yan, Fei Ren, Jong-ook Suh, Annie Huang, and Tiffany Fanyi Ouyang are acknowledged. Also included are the work of several post-docs (Grant Pan, J. W. Jang, Everett C. C. Yeh, Kejun Zeng, J. W. Nah, and L. Y. Zhang) and M. Sc. students (Wang Yang, Ann A. Liu, Jessica P. Almaraz, Quyen Tang Huynh, Xu Gu, Rajat Agarwal, Joanne Huang, and Jackie Preciado). I would like to acknowledge the generous support and to thank these students and post-docs. It is their dedication and hard work that made this book possible.

Many outstanding contributions to solder research have been made by other researchers. Since this book is an introduction to solder joint technology and covers only a small part of the literature on the subject, I was unable to include much of the published work on solder joints in the literature. I apologize to those colleagues whose work is not included here. Still I hope the book may serve as a stepping stone to reach out to a much broader field of solder and as a reference to future R&D in the field. Indeed, much work on the reliability of Pb-free solder joints remains undone.

While this book is intended for engineers and scientists working on solder joint technology in the electronic manufacturing industry, it might be used as a reference book in a course on reliability science of electronic packaging technology for seniors and graduate students. Because very few textbooks on the subject of electronic packaging technology and reliability science are available, I include in the Appendix, the derivations of the diffusion coefficient in vacancy mechanism of diffusion in a face-centered-cubic lattice, the growth and dissolution equation of a spherical particle in the ripening process, and Huntington¡¦s electron wind force in electromigration. They are convenient references for analyzing the basic kinetic behaviors of solder joints discussed in this book. I would like to acknowledge that the last derivation on electron wind force has been taken from the lecture notes of Professor A. M. Gusak at Cherkasy National University, Ukraine. He has been very helpful on the kinetic analyses presented in this book, for example, irreversible processes. Also I am grateful to Dr. Yuhuan Xu at UCLA and Prof. Yiping Wu at Huazhong University of Science and Technology, China, for helpful comments on drop test in Chapter 11. I would like to thank Prof. Chih Chen, National Chiao Tung University, Hsinchu, Taiwan, ROC and Prof. Cheng-Yi Liu, National Central University, Chungli, Taiwan, ROC, for a critical review of the book, and to Mr. Jong-ook Suh at UCLA for preparation of all the figures and Miss Fanyi Ouyang for revision corrections. Finally, I would like to thank Prof. Y. C. Chan of the Department of Electrical Engineering, City University of Hong Kong, and Prof. Weijia Wen of the Department of Physics, Hong Kong University of Science and Technology for hosting my two-month visit to Hong Kong so that I could concentrate on finishing the final version of the book.